Project

Rocket Nozzle

BURPG · Boston University · Fall 2024

A team project with Boston University Rocket Propulsion Group (BURPG) to design and 3D print a rocket nozzle, plus the electronics to hot-fire it. Flow calculations sized the nozzle, SolidWorks FEA showed a minimum factor of safety of 2.1 at an 80 psi chamber pressure, and the printed nozzle was then hot-fired on a test stand.

Role: Team member (team of 5): design, CAD, FEA, 3D printing and testing
Tools: SolidWorks, SolidWorks FEA, 3D printing (ABS-like resin), Arduino Uno

Mechanical
Simulation

The 3D-printed translucent resin nozzle standing on a table after its hot-fire test, with a scorched, discolored bore and a chipped exit rim — The 3D-printed nozzle after its hot-fire test.

The hot-fire test, filmed in the fume hood.

Problem

The brief was to build a rocket nozzle from a set of initial constraints, along with the electronics needed to hot-fire it and record thrust. The test was a static firing, so mass wasn't a design driver.

Approach

We built a spreadsheet from the textbook equations to size the nozzle from chamber pressure, assuming isentropic flow. It gave a specific impulse of 274.6 s, an exit velocity of 2,691 m/s, a critical pressure ratio of 0.578 (about 46 psi at the throat with an 80 psi chamber), an area ratio of 6.52, a 7.2 mm throat diameter, an 18.4 mm exit diameter and a 16.8 mm length (an 80% length nozzle).

In SolidWorks the nozzle mates to an adapter on a 1 inch NPT chamber nipple through six 1/4-20 bolts and nuts. The calculated dimensions drive the geometry, and the part weighs 0.04 kg.

For the FEA we built a custom material from the resin's data sheet, taking yield strength as 70% of tensile strength (25.2 MPa). We applied 80 psi chamber pressure and about 46 psi throat pressure, with the six bolt-hole faces fixed.

We compared machining the part on a lathe, which is good experience but slow and impractical for several groups, with 3D printing, which only needs the CAD model but raised a concern about melting during the firing. We chose 3D printing in ABS-like resin. Heat transfer wasn't simulated, so we thickened the throat as a precaution and set the wall thickness at 0.1 in.

The test electronics use an Arduino Uno with a load cell and HX711 amplifier to record thrust, an SD card board to log the data, and a push button that starts the sequence. Relays switch the 12 V solenoid and the e-match, with a diode across the solenoid to block back-current.

For the firing, the chamber and nozzle were mounted on an aluminum-extrusion test stand inside a fume hood, with the load cell in line to measure thrust.

Results

Calculated performance: 35.0 N of thrust at the optimal expansion and 11.8 N at sea level. The FEA gave a maximum von Mises stress of 11.9 MPa against the 25.2 MPa yield strength, and a minimum factor of safety of 2.118, above our target of 2, with most of the part above 3.

We hot-fired the nozzle on the test stand (video above). The printed part stayed intact: afterward its exit rim was chipped and its bore was scorched, which is the kind of heat damage we had flagged as a risk of printing it.

Gallery

A hand holding the fired nozzle up to the camera, showing the flange with its six bolt holes, the scorched bore and the chipped exit rim — The nozzle after the test: the exit rim is chipped and the bore is scorched, but the part stayed intact.

The test stand inside a fume hood before the firing: an aluminum-extrusion frame holding the chamber and nozzle, a load cell, brass valve fittings and wiring — The test stand before the firing, inside a fume hood: the chamber and nozzle on an aluminum-extrusion frame, with a load cell, valve fittings and wiring.

Five team members posing in front of the Boston University Rocket Propulsion Group door, one holding up the nozzle — The BURPG team outside the lab, with the nozzle.

CAD render of the nozzle assembly, labeled: the nozzle (geometry hidden), the adapter, the 1 inch NPT chamber nipple, and the 1/4-20 nuts and bolts — The assembly in CAD: the nozzle bolts to an adapter on a 1 inch NPT chamber nipple.

Exploded CAD view of the nozzle and adapter with six bolts and nuts — Exploded view: the nozzle and adapter are joined with six 1/4-20 bolts and nuts.

Engineering drawing of the nozzle with a section view, detail views and critical dimensions — Part drawing, dimensioned from the calculated throat, exit and length values.

Engineering drawing of the nozzle and adapter assembly with a section view, a detail of the sealing region, an isometric view and an exploded view — Nozzle and adapter drawing: section view, sealing detail and exploded view.

Section view of the nozzle showing pressure loads as red arrows and fixed bolt-hole faces in green — FEA setup: 80 psi chamber pressure, about 46 psi at the throat, and the bolt-hole faces fixed.

SolidWorks von Mises stress plot of the nozzle, with the highest stresses around the bolt holes — Von Mises stress: 11.9 MPa maximum against a 25.2 MPa yield strength.

SolidWorks factor of safety plot of the nozzle with a minimum of 2.118 near a bolt hole and most of the part above 3 — Factor of safety: 2.118 at the minimum, with most of the part above 3.

Circuit schematic of the hot-fire electronics: Arduino Uno, load cell amplifier, SD card board, relays, solenoid and e-match — Hot-fire electronics: Arduino, load cell, SD card logging, and relays for the solenoid and e-match.